226 
On the Construction 
rMonthly Microscopical 
L Journal, April 1, 1869. 
Fig. 1. 
Fig. 2. 
approaches to the inner side of the concentric Hght-rings, as in 
Fig. 1, it is termed " outward coma," and indicates that the front 
incident surface of the back triple is 
too convex. If, on the other hand, 
the bright spot is on the outer side of 
the rings, or next the margin of the 
field of view, there is " inward coma," 
which shows that this same surface is 
too flat. I have previously remarked 
that this curve has a powerful effect on 
the flatness of field and perfection of 
the oblique pencils, and for these no 
other correction is generally requisite 
than an alteration in this radius. 
Before the glasses are finally ce- 
mented in their cells they should be carefully tested for centering ; 
for this purpose a very minute globule is selected, and placed exactly 
in the centre of the field. If the bright 
spot appears eccentric, with the rings 
thus (Fig. 2), the pair of lenses which 
occasion the error should be shifted on 
each other while warm enough to cause 
the Canada balsam by which they are 
cemented together to yield, till on re- 
peated trial the error is corrected. This 
is important, as the least fault of center- 
ing materially impairs the performance 
of an object-glass. But with the pre- 
cautions that I have adopted in the con- 
struction, to be hereafter explained, errors of centering cannot occur. 
There is yet one other globule test for object-glasses, to indicate 
accuracy of workmanship, or whether the lenses are worked to true 
spherical surfaces. If the rings from 
a minute globule appear of an irregular 
wavy outline, as shown by the annexed 
cut (Fig. 3), either approximating to a 
polygon or triangle, it shows that one 
of the surfaces at least that refracts 
the rays is of this form. Such work- 
manship is inexcusable, and those that 
cannot avoid it had better let glass- 
grinding alone. 
Finally, there is an appearance that 
I have sometimes seen in our best 
object-glasses, when focussed away 
from a globule, viz. " Newton's rings ; " this shows that in the 
Fig. 3. 
m 
